Any ideas where to start on a fix?

 

If you did a search, you would have found this,
https://www.hummerforums.com/forum/h...-update-26219/

I solved my flickering with carbon conductive grease on the wiring harness connector on the back of the head unit. This has to be the problem and cure, because it has happened too many times, to others, to be something else
Check the above link and read

 

I will pick some up and see what happens

 

Good Luck
Remember, Carbon Conductive Grease, nothing else !!
You can PM me if you want specifics, before you attempt anything